Practical applications of hig h-temperature superconductors

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by R. C. Woods

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A recent colloquium held at Savoy Place and ovganised by the Electrfomagnetics Professional G ~ w p ofthe IEE examined a number ofpractical applications ofthe newfdmily of ‘high-temperature’ supevconductors. This highly successful event was co-sponsored by a number ofother IEE Pvofessional Gifoups, as well as the Institute (f Physics. The colloquium aroused a great deal of interest, with an unusually latge numbes of attendees, demonstrating the irnpoi!tance of these new developments f o v engineers thinking about system for the next century; two ofthe speaker3 were inteiviewed by the BBCfor broadcasting in a science propamme shortly dtefwavds. Here, the Chairman ofthe colloquium summarises the cumnt state-of-the-art in the use of these new materials in engineering applications.

required. However, in 1986, Bednorz and

Introduction 1 and 0.75 and y > 0, and giving critical temperatures in the 30 K range, and by 1987 the first so-called

uperconductivity or the phenomenon of classical ‘high-teniperature’ superconductor (HTS) complete absence of electrical resistance under material, Y1BazCujO.i-s, or YBCO-I 23 for short, had certain conditions at low temperatures, has been been discovered2. For YBCO-123 the critical S known ever since Kammerlingh Onnes (work- temperature is around 92 K (-181°C); for

ing at the University of Leiden in the Netherlands) BizSrzCaCu20, it is around 108 K (-165OC);

E temperature A B C D

liquefied helium and subsequently, in 1911, measured the resistance of soiiie saniples of elements when cooled to 1.7 I< (= -272OC) (see Fig. 1). For all superconductors there is a so-callcd ‘critical temperature’ (T,) below which the material conducts with zero resistance, at zero applied magnetic field, and above which there is the conventional resistance to current flow. Until 1986, it appeared that superconductivity would always be the preserve of the low-temperature physicists, with temperatures somewhat below 30 K (-243OC) apparently always

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for T12BazCazCu3Oi0-s, T, is approximately 127 K (-1 45°C)3, and the phase in the HgBa.Ca,,-iCu,,- 02r,+2+~systeni with iz = 3 has T, = 135 K (-138°C) at ambient pressure‘. (Reports in the late 1980s of critical temperatures niuch closer to, and even above. room temperature arc now widely discounted; usually the)- were due to misinterpretations of unimportant and minor artefacts in plots of resistivity against tempera- ture.) There is also a ‘critical magnetic field’ which: when exceeded, causes the material to revert to the ‘normal’ or noli-superconducting state. The value of this critical field is zero at the critical temperature. but increases ac the temperature reduces below the critical temperature.

In fact it appears that superconductivity is the rule rather than the exception; the majority of pure metallic elenicnts, and many alloys, are superconductors at veiq- low temperatures (around -260°C and below). What makes the new HTS materials so exciting, from the physicists’ point of view, is that they are non-metallic ceramic oxides (always, apparently, containing copper) ; far from being good conductors (like metals) above their supercopducting critical temperature, they have much in conmon with semiconductors (such as silicon) when not superconducting. From the engineers’ point of view, their superconducting critical temperatures are much higher (around -1 80°C) than those of the previously known metallic super- conductors.

This seemingly small difference is highly important technologically, because it means that instead of a veiT expensive liquid helium-based cooling system or cryostat (necessary for operation below around 30 K), a very much simpler and cheaper cryostat suitable for liquid nitrogen at 77 K (its boiling temperature at a pressure of one atmosphere, -196°C) can now be used. Liquid nitrogen is widely ured in many industrial processes; it is an extremely cheap industrial com- modity, easily obtainable from the air (and auto- matically recycled once used!). It is also considerably easier to use than liquid helium because its specific heat capacity (and hence ‘cooling power’) is much greater than that of liquid helium (for fundamental quantum- mechanical reasons). The critical temperature of YBCO-123, although greater than 77 K, is actually rather too close to 77 K for comfort, and operating so close to the critical temperature is unlikely to be attempted in serious applications; nevertheless, most of the early engineering work on the new materials has been performed on YBCO because it typifies the HTS family of materials. The newer materials in the BiSrCaCuO or TlBaCaCuO systems, or materials developed cven more recently, are likely to be used in practice.

As is characteristic of all new research fields, there wa3 an initial rush to publish results, sometimes of high quality and sometinies not. These new results helped to spawn a significant number of new research journals. Now that this transient has passed, however, the field is entering a inore mature phase in which there are very

real and clearly defined opportunities for designers to make highly important and challenging innovations in electronic and electrical engineering.

Low-power devices

Superconductors offer many possibilities for the design of microelectronic devices and other components used in high-speed electronics, through their novel physical and electronic properties. The components where HTS have the potential to make a significant impact include resonators and delay lines of various kinds, antennas, and solid-state devices for fast switching, digital logic, and other applications.

In antenna design the potential advantages given by HTS technology are that the resistive losses in the antenna material are, if not eliminated, significantly reduced. The first antenna using HTS technology was developed in 1989’ and offered a modest improvement in gain over an equivalent copper antenna cooled to the same temperature. (Note that the correct comparison must always be made with copper at reduced temperature, because although it is not super- conducting at 77 K the resistivity of copper at this temperature is significantly less than at room tempera- ture.) In all superconductors, complete absence of resistance only occurs strictly at DC, and at high fiequencies (KF and microwaves) the improvements possible are not as spectacular as at low frequency or D C operation. Nevertheless, it has now been established that the new materials are capable of providing a significant enhancement of performance over the conventional non-superconducting equiva- lents. In principle, similar advantages can be gained xvhen using HTS materials for resonators and delay lines. However, the improvements possible may perhaps be more elusive in practice, because in these applications the surface conductance in conventional (non-superconducting) designs is often enhanced by using gold plating, which has a very small sheet resistance. At high frequencies the well-known ‘skin- effect’ causes the significant signal currents in a typical cavity resonator or stripline to remain near the surface of the conductors, and therefore to be confined within the low-resistivity plating. Nevertheless, HTS components are starting to appear; in a typical design‘ a microstrip delay-line matched to 50 !2 produces a 3 ns delay with low dispersion over a bandwidth of 5 GHz. The microstrip is produced by photo- lithography in a coplanar (non-inductive) double-spiral using a L d O 3 substrate, particularly suitable for high- frequency devices, to avoid micro-shorts.

The initial work on antennas was performed using bulk YBCO wires, but much of the work on high- frequency resonators and high-speed switching devices now depends upon the production of highly uniform thin and thick films of HTS. The preferred techniques here are R F sputtering (using a plasma generated by an intense RF electric field, for production of very thin films), laser ablation (using high-power radiation from

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an excinier laser incident upon a target of bulk HTS material, for thicker films but with not quite the same absence of irregularities or so-called ‘boulders’ as can be achieved in the best sputtered films), and conventional screen printing.

Many laboratories world-wide have now produced ‘SQUIDs’ (Super- conducting Quantum Interference Devices), fabricated from the original ‘low-temperature’ superconductors (LTSs), for high-precision measure- ments of weak magnetic fields. These devices rely on the fact that the total electric current through two parallel superconducting ‘weak links’ is (for quantum-mechanical reasons) a periodic function of the magnetic flux through the superconducting loop containing the ‘weak links’ (the device corresponds to the superconducting analogue of the well-known ‘Young’s slits’ experiment in optics). As well as a critical temperature and a critical magnetic field, there is also a ‘critical

I 1 suDerconductor

step

a b

Fig. 2 SQUID magnetometers: (a) schematic arrangement; (b) typical actual construction using HTS materials8. The HTS film is deposited over a shallow etched step in the substrate and is therefore discontinuous at this step; the ‘weak links‘ are produced by coupling of the HTS materials on each side of the step using a silver or goldlsilver alloy thin film. The patterning of the films is obtained by photolithography and ion-beam etching

- current-density’ beyond which any particular superconductor is no longer superconducting. The ‘weak links’ are regions of very small cross-sectional area through which any supercurrent has to flow, and where accordingly the critical current-density value is exceeded locally. The maximum current flow occurs at flux values exactly equal to an integer multiple of the ‘magnetic flux quantum’, @n = h/2e = 2 x Wb, and so measuring the current gives a precise measure of the magnetic field. SQUIDs have applications in precise magnetometry, medical electronics (for example, in detecting and measuring the minute magnetic fields produced by brain activity), and other areas. Similar devices are now starting to emerge made from the new HTS materials (see Fig. 2). The HTS versions generally do not have a performance as good as the LTS equivalent (particularly concerning their noise levels), but as usual are considerably easier to use in practice because of their considerably relaxed refrigeration requirements. There is an interesting ‘spin-off advantage as well, because their simplified cryogenic operation enables HTS SQUIDs to be used physically much closer to any sample being measured. An LTS SQUID, with much care and awkwardness in operation (for example, very high helium refrigerant boiling rates), can be made with as little as 2 mm ‘stand- off, but this figure and even smaller values are easily achieved with an HTS SQUID. Although still in their infancy, HTS SQUIDs may soon replace their LTS counterparts in all but the most exacting applications’.

Although work using HTS materials is certainly not as advanced as that using LTS materials, a typical exaniple of the fabrication and trial of a high-speed integrated LTS component9 having low-power dissipation points towards future developmental goals

in this area using HTS materials. At the moment, HTS logic components are limited to simple gates and switches, mostly based upon tunnelling links, but presumably it will only be a matter of time before comparable large-scale integrated HTS components can be produced using ‘foundry’ type standardised processes.

Magnetic-field production

Once again, conventional superconducting technology can perhaps point the way towards the expected immediate and most practical uses of the new materials. The two major current industrial uses of conventional LTS technology are both based upon the production of high magnetic fields: magnetic separation (of ferrous from non-ferrous materials) and medical resonance imaging (MRI). In MRI a magnetic field of exceptional uniformity (homogeneity) over a large physical volume is necessary. This is used to cause high- frequency gyroscopic rotation or ‘precession’ of the magnetically active nuclei in atoms within the human body, which can then be detected by sensitive specialised receivers. Different types of tissue produce cfifferent types of signal, giving a valuable diagnostic tool. (High field uniformity is required because the nuclear precession frequency is precisely determined by the magnetic field, and in order to detect the effect it is necessary that all the nuclei precess in phase.) The high uniformity is achieved in practice by using special coil configurations and also by using only a relatively small fraction of the volume in which the available magnetic field is generated.

The advantage that superconducting technology offers these applications is that once the magnetic field

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Fig. 3 The ‘Meissner effect‘, or the expulsion of all magnetic flux from a superconductor (producing a repulsive force on a strong magnet placed close to the superconductor), demonstrated by the levitation of a SmCo permanent magnet above a sample of YBCO at 77 K

has been established by passing a large supercurrent through a suitable superconducting coil, no further expenditure of enerby is required other than that needed by the cryostat to keep the coil below its critical temperature; if the two ends of the coil are connected with a superconducting link, the supercurrent continues to flow indefinitely The source of the current (perhaps a generator) and the distribution wiring are no longer required and can be used for other purposes until the next time that the superconducting magnet must be energised. By contrast, using a conventional electromagnet, energy is constantly needed to overcome the winding resistance in the electromagnet and the generator and in the distribution wiring (as well as the cooling system needed, in this case, to prevent the temperature of the magnet rising too far above ambient temperature). In practice this means that for magnetic fields below around 8 T, convcntional technology is usually optimal, whereas at higher fields the superconducting alternatives must be considered. LTS electromagnets are well-established in research laboratories world-wide, but HTS materials offer the prospects of increased efficiency and reliability as well as reduced cost, compared to the LTS alternatives. The increased reliability arises from several reasons, including the increased thermal stability of materials operating at 77 K rather than 4.2 K (using a helium cryostat) due to their increased specific heat capacity, and also the siniplificd cryostat design possible.

The value of the critical current-density is reduced when there is a niagnetic field present, and so clearly this represents a potential major limitation of ruper- conducting electromagnets. By operating a helium cryostat at 4.2 K, the critical current-densities at high fields are increased (to around 10‘‘ A &) over their values at 77 K (of the order of 10’ A I+) arid so one approach is to use an HTS auxiliary solenoid as an integral part of a ‘conventional’ LTS solenoid also operating at 4.2 K. This can help to overcome the

poorer performance of the traditional LTSs in fields above 15 T. Attaining large current densities in the long lengths of superconductor necessary for a large electromagnet has caused problems in the past, although these are now being overcome. The alternative is to operate the new materials at 77 K and to tolerate the markedly reduced current-carrying capacity. and hence maximum field that can be produced, at this temperature. Applications such as magnetic separation can benefit from the simple advantages of ease of cooling and the effectively zero power input requirement.

Other significant uses of high magnetic fields include magnetic levitation; the ‘MagLev’ shuttle at Birmingham airport is a well-known example of magnetic levitation on a large scale using conventional electromagnets. In this type of application the magnetic field must be adjusted by a closed-loop feedback system in response to the variable instantaneous separation distance between the magnet pole-pieces. This requires the use of large power amplifiers within the control loop. which may be reduced in size and weight if superconducting magnetic levitation could be used. Direct superconducting levitation using the Meissner effect (see Fig. 3) has not so far been used in large-scale levitation projects. In esoteric applications, such as the magnets for guiding high-energy particle beams at the European Nuclear Research Centre (CERN) in Geneva, conventional superconducting technology is already used on an extremely large scale. Thc new superconducting technology is set to revolutionise these kinds of applications by significantly reducing the power and expenditure necessary.

High-power applications

Complete absence of rcsictancc wunds like the power engineer’s dream, and indeed niaiiy potential apphcationt of the new HTS technology are in

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electrical power distribution. Fortunately, liquid nitrogen is a good electrical insulator (at least comparable to air at ambient temperature, and superior to liquid hcliuin). So far, most designs produced have used superconducting stator windings only and so the advantages given by the new materials are similar to those in magnet design. The complexities of producing a rotating cryogenic system have so far precluded any full-sized attempts at making superconducting armature windings, although the simplifications of the cooling requirements of the new HTS materials over the conventional superconductors mean that this may only be a matter of time. Even considering only an HTS stator design, cost savings (over conventional technology) of the order of A20M over 20 years for a 1000 M W generator have been projected. High-power transformer design is projected to offer similar improvements, of the order of a few tenths of one percent in efficiency.

The use of superconducting technology has also produced several types of fault current limiting devices. These devices are used in high-power installations to limit the current flow under fault conditions in order to restrict the magnitude of any damage caused by the fault. There are several types of fault current limiters, each with its own mode of operation. The simplest relies upon the critical current-density (above which a superconductor is not superconducting) being a well- defined property of a superconducting material. If a superconducting component is simply placed in the current path of the circuit to be protected, then by suitable choice of the cross-sectional area of the superconductor it can be made to revert to a nornial (non-superconducting) state when the appropriate maximum current level is exceeded. LTSs are more suitable than HTSs for this kind of device.

This type of fault current limiter is not satisfactory in actual use because of the potential catastrophe that can be caused by sudden 'quenching' or disappearance of the superconductivity and consequent sudden heat production. This problem has been recognised since the early days of conventional superconducting electro- magnets; what appears to happen is that a small localised region of the windings changes from the superconducting to the normal (resistive) state spontaneously. The precise reasons for this are not particularly clear but may depend upon a non-uniform temperature distribution within the superconductor causing localiscd 'hot-spots' which eventually become so 'hot' that the critical temperature is exceeded and they are no longer superconducting. At this point, such a large current is flowing in a typical

magnet rises so rapidly that the refrigeration system is unable to cope. Therefore, the entire magnet loses its superconductivity and the circulating current, producing the magnetic field, reduces to zero in a very short time. All the potential energy previously stored in the magnetic field will then be released quickly, usually by vapourising a large quantity of refrigerant (and often the electromagnet as well) with generally spectacular results.

In addition to this possibility, the large amount of power generated in the simplest type of fault current limiter under overcurrent conditions is increased by operating too close to the critical temperature, and so more sophisticated designs have now appeared"'~". One design relies, again, upon the ease of producing a large magnetic field by using superconducting means, and uses a constant supercurrent to drive a ferro- magnetic core well into magnetic saturation. This core carries, in addition, conventional windings through which is flowing the current to be limited. If the current through these windings exceeds a certain value, the peaks of the AC excursions in one direction will bring the core out of saturation. While the core is out of saturation, the self-inductance of the windmgs on the core w d increase dramatically and so cause a reduction in the current flow. The operation of this device, in practice, is complex because it relies upon the magnetic core being highly non-linear; this type of limiter is always operated in pairs (connected in opposite phase) in order to produce current-limiting characteristics symmetrical about zero. Another design (see Fig. 4) relies upon the superconductor acting as a perfect magnet screen (the Meissner effect again) between the conventional (non-superconducting) winding and a ferromagnetic core. Under normal

iron

circuit -magnetic

large electromagnet that even a small produces a large amount Of

power, and the temperature of the whole

Fig. 4 value of the HTS material to govern the current value that causes a sudden increase in the inductance presented to the external circuit

A typical design of fault current limiter1' using the critical field

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condxions, the inductance seen by the external circuit is very low because the magnetic core is efficiently screened from the windings. However, under fault conditions, the critical field of the superconductor is exceeded and the effective inductance rises dramatically, reducing the current flow.

These types of design have certain advantages over the simplest resistive type, because the superconductor doer not carry the current of the circuit being protected. Also, by limiting the current inductively rather than resistively, power dissipation problems are minimised. Accordingly, the UK electricity supply industry is currently working on plans for an improved design of current limiter using supercoriductor technology. These will primarily be intended for medium-power applications, for example in local power distribution at the 11 kV level (250 MVA rating), and may need to be used specifically \vhen other superconducting equipment has been deployed in the same circuit.

Power distribution is another area of interest for electrical engineers working with HTS. Here the cost of refrigeration and thermal insulation of the HTS must be balanced against the resistive (12R) losses (as well as perhaps the cost of cooling) of conventional cables; at 50 or 60 Hz the resistive losses in the HTS material are negligible. Conventional underground cables very often are liquid-cooled (to prevent overheating) using oil, and the advantages gained by using liquid refrigerant instead of cooling oil include a considerable reduction in the risk of fire and pollution caused by leaks. Generally spealung the new HTS cables are designed to be retro-fittable into existing power cable ducts and have been demonstrated to be econonic at around the 500 MVA power level and above, depending upon the niarket conditions. HTS wires incorporating silver cladding are one promising avenue being explored now. In Europe, particularly, DC ‘ring-mains’ around cities or between countries have been identified as one potential area where HTS cabling will be of significant benefit. Because existing cable ducts are of standard sizes, there is considerable pressure on the cable manufacturers to produce cables with higher critical current-densities, hence carrying higher currents in a given cable cross-section”.

Conclusions

The new HTS technology has very rapidly established itself as a leading contender for consideration in a wide variety of applications where previously the high cost of cooling superconductors has precluded their widespread use. This is due in part to the extremely large nuniber of active and talented research and development scientists and engineers working in the HTS field world-wide, in part to the revolution caused by the unique and previously unsuspected properties of the HTS materials that have been developed recently, and to the demands of the industry not being fully satisfied by existing materials.

Acknowledgments

At the time of writing, the author was with the Laboratoire de Physique de la Mati?re, Institut National des Sciences AppliquCes de Lyon (URA CNRS 358), 69621 Villeurbanne Cedex, France, on special leave fi-om the Department of Electronic and Electrical Engineering, University of Shefield. This paper was written as part of the research programme at CRELYMO (the Lyon research centre for micro- optoelectronic materials and devices).

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8 IEE: 1996

Dr. Woods is with the Department of Electronic and Electrical Engineering, Univercity of Sheffield, Mappin Street, Shefield, S1 3JD, UK.

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